1. Field of the Invention
The present invention relates to a triaxial force sensor and a triaxial force detection method preferably used for detecting forces in the X-axial direction, the Y-axial direction and the Z-axial direction that act on a protruding load action element arranged in the center of one of the surfaces of a strain causing plate.
2. Description of the Relevant Art
Conventionally, a triaxial force sensor composed of a detection element comprising a protruding load action element arranged on one of the surfaces of a strain causing plate and multiple strain sensitive elements arranged on the other surface of the strain causing plate for detecting forces acting on the load action element in the X, Y and Z-axial directions, and a signal processing unit that processes signals from the strain sensitive elements of said detection element is known in Japanese Patent Application Laid-Open Nos. 2004-239621, 2000-267803 and 2004-212047 (U.S. Pat. No. 6,993,982).
A three-dimensional pressure sensor disclosed in No. 2004-239621 is provided with a rod-like stud, a base member for securing the pressure sensor, a bending strain detection plate connected between one end of the rod-like stud and the base member, and a plurality of strain sensors arranged on the surface of the bending strain detection plate; the bending strain detection plate has a cross part wherein the rod-like stud is connected to the center of this cross part; the base member is connected to the periphery of the cross part; the strain sensors are arranged on the surfaces of an upward arm, a right-pointing arm, a downward arm and a left-pointing arm, which form the cross part, respectively; and a signal processing circuit conducts pre-determined arithmetic processing using a strain sensor resistance value when there is no contact pressure applied to the end of the rod-like stud and another strain sensor resistance value when the contact pressure is applied, and outputs the processed contact pressure value.
However, in the three-dimensional pressure sensor disclosed in No. 2004-239621, the number of strain sensitive elements to be used is increased. In this case, a total of sixteen strain sensitive elements are required, causing a cost increase due to an increase in the number of components. Since each strain sensitive element needs to be arranged on the surface of the strain causing plate, there is a limitation to miniaturize the detection unit. In particular, when the sensor is mounted to a fingertip of a robot hand having the same size of a human hand, a corresponding size reduction is required. However, the invention in No. 2004-239621 cannot meet this requirement.
Further, the pointing device disclosed in No. 2000-267803 is equipped with an electrode land in the outer circumference and a post in the center, and it comprises a thin polygonal or circular ceramic substrate provided with four resistors midway between the outer circumference and the center, radially relative to the post, and a pedestal mounting the substrate. The electrode land in the outer circumference of the substrate is electrically connected and fixed to the pedestal by soldering creating a gap between the substrate and the pedestal. When the post is pressed, the substrate bends with the soldered portions of the outer circumference working as supporting points, the resistors are strained and the pressing force on the post is detected. The lower end of the post is provided with a thick pedestal contacting the substrate, and the resistors are arranged at the back side of the substrate which is pressed by the pedestal when the pressing force is applied to the post.
In addition, the stress sensor disclosed in No. 2004-212047 is a stress sensor where strain gauges formed by resistive elements are arranged at four points located on one of two straight lines intersecting perpendicularly at the center of a sensor effective region of a substrate plane, and substantially equidistant from the intersection. The post is fixed or integrated to the substrate plane so as to substantially match the center of the sensor effective region on the substrate plane with the center of the post bottom face. The direction and magnitude of the stress can be determined from a change in the resistance value of the resistive element due to stretch, contraction or compression caused by the stress applied to the post wherein the post bottom part has a protrusion acting on the strain gauge when the stress is applied to the post.
However, although the number of strain sensitive elements is reduced to four in both the pointing device disclosed in No. 2000-267803 and the stress sensor disclosed in No. 2004-212047, the detection method for a force in the Z-axial direction is not disclosed. In other words, in No. 2000-267803, four resistors and electrode wires are arranged on the back surface of the substrate, and even though they are connectable as a bridge circuit and can extract an equal output with regard to the stresses in the X, Y and Z-axial directions, the type of bridge circuit configured by the four resistors is not disclosed. Regarding the reference No. 2004-212047, the arrangement of the four resistive elements and conductors on the back surface of the substrate similar to No. 2000-267803 is shown. At the same time, a bridge circuit formed with these four resistive elements is shown. However, while a stress sensor in the X-axial direction and another stress sensor in the Y-axial direction are formed by the bridge circuit, nothing about the stress sensor in the Z-axial direction is disclosed.
In addition, the triaxial force sensors disclosed in Nos. 2004-239621, 2000-267803 and 2004-212047 all have a problem that a disconnection or loose connection easily occurs to a joint of the detection unit, as well. In other words, although it is necessary to attach to the detection unit a lead which is connected to the strain sensitive element and led to the outside, a tension-compression stress often acts on the joint because the strain causing plate where the strain sensitive elements are attached is strained (bent) by an applied force, and a disconnection caused by a fatigue failure can easily occur. Therefore, it is difficult to assure the detection unit reliability.